1. Field of the Invention
The present invention relates generally to an ultrasonic system for detecting the presence of entrained air in a fluid line, and more particularly to a system using a multiple windowing and air volume number weighting technique to determine when a threshold count of entrained air in a predetermined number of windows of material pumped occurs, with the alarm being sounded only when this threshold count of entrained air bubbles is present in the multiple window volume, thus detecting even entrained air while avoiding nuisance alarms.
In the past there have been two primary techniques which have been used to deliver drugs which may not be orally ingested to a patient. The first such technique is through an injection, or shot, using a syringe and needle which delivers a large dosage at relatively infrequent intervals to the patient. This technique is not always satisfactory, particularly when the drug being administered is potentially lethal, has negative side effects when delivered in a large dosage, or must be delivered more or less continuously to achieve the desired therapeutic effect. This problem results in smaller injections being given at more frequent intervals, a compromise approach not yielding satisfactory results.
Alternatively, the second technique involves administering a continuous flow of medication to the patient, typically through an IV bottle. Medication may also be delivered through an IV system with an injection being made into a complex maze of IV tubes, hoses, and other paraphernalia. With drop counters being used to meter the amount of bulk fluid delivered, many medications still end up being administered in a large dosage through an injection into the IV lines, although the medications may be diluted somewhat by the bulk fluid.
As an alternative to these two techniques of administering medication to a patient, the relatively recent addition of medication infusion pumps has come as a welcome improvement. Medication infusion pumps are utilized to administer drugs to a patient in small, metered doses at frequent intervals or, alternatively, in the case of some devices, at a low but essentially continuous rate. Infusion pump therapy may be electronically controlled to deliver precise, metered doses at exactly determined intervals, thereby providing a beneficial gradual infusion of medication to the patient. In this manner, the infusion pump is able to mimic the natural process whereby chemical balances are maintained more precisely by operating on a continuous time basis.
One of the requirements of a medication infusion system is dictated by the important design consideration of disposability. Since the portion of the device through which medication is pumped must be sterile, in most applications of modern medication infusion equipment some portions of the equipment are used only once and then disposed of, typically at regular intervals such as once daily. It is therefore desirable that the fluid pump portion of the infusion pump device be disposable, with the fluid pump being designed as an attachable cassette which is of inexpensive design, and which is easily installable onto the main pump unit.
It will be perceived that it is desirable to have a simple disposable cassette design to minimize the cost of construction of the cassette, using the minimum number of parts necessary in the design of the cassette. The design of the cassette must be mass producible, and yet result in a uniform cassette which is capable of delivering liquid medication or other therapeutic fluids with a high degree of accuracy. The cassette should include therein more than just a fluid pump; other features which have formerly been included in peripheral devices may be included in the cassette. Such a system has been disclosed in all of the above-identified previously filed related applications.
An essential function of a medication infusion system is to avoid the infusion of fluid containing more than a minimal amount of air bubbles therein. Although steps may be taken to minimize the possibility of air bubbles being contained in a fluid which is to be infused to a patient, it is essential to monitor the fluid line before it reaches the patient to ensure that air bubbles remain in the fluid which is to be infused are detected. The detection of air bubbles in all fluids which are to be infused is therefore a critical design requirement.
One type of system which can detect air-in-line uses an optical detection system, which is the subject of U.S. Ser. No. 128,121, entitled "Air-In-Line Detector for a Medication Infusion System," which application is hereby incorporated herein by reference. Another type of system for detecting air-in-line is an ultrasonic system, which is the subject of U.S. Ser. No. 403,512, entitled "Ultrasonic Air-In-Line Detector for a Medication Infusion System," the parent of the present application.
Ultrasonic air-in-line detectors utilize an ultrasonic transmitter on one side of a fluid line and an ultrasonic receiver on the other side of the fluid line. Fluid is a good conductor of ultrasonic energy while air or foam is not. Accordingly, if there is an air bubble in the fluid line between the transmitter and the receiver, the signal strength will be greatly attenuated, and the presence of the bubble will be indicated. Ultrasonic air-in-line detectors are capable of detecting even very small air bubbles in the fluid line.
It should be noted that in most patients it is not harmful to pump very small amounts of air intravenously. Only a few patients can tolerate no air introduced into their venous systems, such as neonates, pediatrics, and those patients having cardiac septal defects. In such cases where even small bubbles may be harmful, air eliminating filters may be used. Other than when infusing fluid into such patients, or performing an intra-arterial infusion, the introduction of a very small quantities of air is not believed to be particularly harmful.
Thus, it is desirable for an ultrasonic air-in-line detection system to alarm only during the presence of significant air in the fluid line. Nuisance alarms, which occur frequently in some pump designs due to the virtual inevitability of small bubbles being present, are to be avoided.
In the parent case to the present application, an ultrasonic air-in-line detector is used to detect all air bubbles contained in the fluid line leaving a disposable cassette containing a fluid pump. A signal from the air-in-line detector circuitry indicates either the presence or absence of air bubbles contained in the portion of the fluid pathway covered by the ultrasonic sensor. The system functions to evaluate the present indication of the presence or absence of air bubbles together with a past history of the presence of air bubbles during a whether sufficient air bubbles are present to initiate an alarm and to shut down the pump.
The window is a volume less than the total volume of the outlet tubing, to enable an alarm and pump shutdown to occur before air can reach the patient. If during the pumping of the last window volume quantity passing the ultrasonic sensor less than a threshold amount of air exists, the system is allowed to continue operating. If, on the other hand, the threshold amount of air is detected during the pumping of the last window volume quantity, the system is shut down and an alarm is sounded.
The system automatically remembers the volume of air while that air is contained within the window. When the air has been pumped beyond the window, it is automatically "forgotten" since it is no longer within the window. In this manner, the system will be able to continue pumping fluid even though a tiny amount of air is contained in the fluid being pumped, thus avoiding nuisance alarms. However, the threshold is set low enough so that any significant quantity of air will be quickly detected and acted upon.
An analysis of the operating conditions of an infusion set indicates that there are four possible conditions regarding the presence of air in the infusion set. First, if the infusion set is correctly primed and used, and if there are no structural defects in the infusion set, no significant amount of air will be present. In clinical use, it is expected that the infusion system will not produce any alarm indicating the presence of air in the system under these circumstances (no false alarms). The system disclosed in the parent to the present application does an admirable job under this first condition.
Secondly, the fluid contained may empty and the infusion set run dry. In clinical use, the infusion system air-in-line detector must detect the air and stop pumping before the air can reach the patient. The detection of such a large amount of air is a safety function present in most systems; in order to be a viable infusion system, the system must always alarm on an empty container condition. Here again, the system disclosed in the parent to the present application does an excellent job.
The third possible condition is when a single bubble is introduced into the infusion system. A single bubble of sufficient size must be detected by the infusion system and cause the pumping system to shut down. While the infusion system must operate in such situations, they are not typically encountered in clinical applications. Once again, the system disclosed in the parent to the present application does an outstanding job.
Finally, the fourth condition is when air is entrained in the fluid being pumped through the infusion system. In this situation, the infusion tubing between the infusion pump and the patient contains a mixture of air and fluid. Typically, small air bubbles of 10 to 50 microliters are present in the tubing, separated by relatively large volumes of fluid. While each individual bubble is of no concern, the cumulative volume of air may be sufficient to warrant an alarm and cessation of pumping.
This situation is generally caused by a small leak in the fluid path. For example, such a situation may be caused by a pinhole leak in the fluid tubing, a crack in a Y-site, a leak in a cassette due to faulty ultrasonic sealing, or a leaking cap seal. Previously known air-in-line detection systems are not capable of accurately detecting entrained air in the fluid line. Even the system disclosed in the parent to the present application does not do a good job of detecting entrained air; if the system is set to detect entrained air accurately, it will also produce an unacceptable number of nuisance alarms.
It is therefore the primary objective of the present invention to provide an air-in-line detection system which will accurately detect entrained air in the fluid line. The system must not be vulnerable to false alarms due to the presence of a tiny amount of air detected in the fluid path. The air-in-line detection system of the present invention must be capable of detecting even very small air bubbles in the fluid line of a disposable cassette near the output end of the cassette, after the pumping operation has been performed, even when these bubbles are separated by relatively large volumes of fluid. In addition, the system of the present invention must intelligently discriminate between a situation in which only isolated entrained air bubbles are present and the situation when a significant total amount of entrained air is present in the fluid line, not alarming in the former case and always alarming in the latter case.
Several other additional features are desirable in the design of a cassette and a main pump unit making up an air-in-line detection system. Examples of such features are the ability to detect air bubbles whether the flow rate of the fluid in the cassette is fast or slow, and the ability to detect air in the fluid line even when the interior of the fluid line remains coated with fluid. The system must be capable of accurately and effectively detecting air bubbles in any type of fluid which may be infused, whether the fluid is clear or opaque, as in the case of lipid solutions.
The system must accomplish all these objects in a manner which will retain and enhance all of the advantages of reliability, durability, and safety of operation. The system of the present invention must provide all of these advantages and overcome the limitations of the background art without incurring any relative disadvantage. All the advantages of the present invention will result in a superior medication infusion system having a number of advantages making the system a highly desirable alternative to systems presently available.